The superiority of air spring suspension on railcars have been recognized for years. One reason for this is that air springs provide a better vertical ride and is quieter than other types of suspension systems involving mechanical springs and parts.
One of the problems involved in the use of air springs is that such air springs have a very low lateral spring rate. An auxiliary lateral spring arrangement must generally be used if a quality lateral ride is desired.
Another problem with air springs is that they may lose air and collapse. Rubber blocks are often installed inside the air bags at some specified distance under the car body. This would be an acceptable solution if the car is moving slow. However, the vertical spring rate of the rubber block is so high that at high speeds the vibration of the car becomes intolerable. At the same time, this rubber block offers very little lateral spring effect again resulting in an uncomfortable car ride. Other types of emergency springs have been used in case of air spring failure, but generally the use of such emergency springs have involved either an uncomfortable ride or required that the car be moved at a relatively low speed. In addition to effecting riding comfort, overall design requirements for new railway cars often require sufficient vertical and lateral spring rates in the event of air bag failure.
Emergency vertical air springs having a non-linear type spring response are known. Such springs have a high spring rate up to the weight of the car, a low spring rate in the range of loading the car and then a high spring rate above the maximum load of the car. Such types of springs alone, however, while providing satisfactory vertical spring rates under emergency conditions, do not provide adequate lateral spring rates.